The present invention relates to a process for removing quinoline-insoluble minute solid impurities from a heavy oil of coal origin or of petroleum origin by the steps of adding an organic solvent to the heavy oil, adding a surfactant to the resultant heavy oil, treating the resultant mixture by stirring to transform the quinoline-insoluble minute solid impurities into a floating substance on the surface of the mixture and removing the floating substance.
As a raw material for producing carbon material, heavy oils of coal origin and of petroleum origin have hitherto been extensively used because of the economical merit that heavy oil is converted to carbon material in a high rate of carbonization for its material cost. However, the allowable conditions of the properties of the heavy oil as a raw material for producing carbon material are very strict, for instance, in the case of the heavy oil of petroleum origin, since the sulfur content of the heavy oil of petroleum origin is generally high, only those of low sulfur content are selectively used. Namely, the range of selection is extremely limited.
On the other hand, in the case of the heavy oil of coal origin, its sulfur content is lower than that of petroleum origin and its rate of carbonization is higher than that of petroleum origin, however, the quinoline-insoluble minute solid impurities which are contained in only a small amount in the heavy oil of coal origin inhibit the graphitization of carbon materials, and accordingly, the heavy oil of coal origin is not desirable as the raw material for carbon material of high quality such as needle coke and that for carbon fiber. In addition, such quinoline-insoluble minute solid impurities are also contained in the heavy oil of petroleum origin although the content is small. The allowable content of the quinoline-insoluble minute solid impurities depends on the use of carbon material, and the so-called quinoline-insoluble component is less than 100 ppm for producing carbon fiber and less than 300 ppm for producing the other carbon material in general standard, the quinoline-insoluble component being determined by the method described later.
Consequently, if the solid impurities contained in the heavy oil of coal origin or of petroleum origin can be effectively removed, the resultant heavy oil can be utilized as the raw material for producing carbon material of high quality, and such a removal contributes largely in cost-reduction of the carbon material.
The quinoline-insoluble minute solid impurities in the heavy oil mean the floating particles of less than 500 microns in representative diameter consisting of carbon and inorganic salts, the floating particles being difficultly separated from the heavy oil or hardly precipitated. In order to separate the floating particles, i.e. the quinoline-insoluble minute solid impurities from the heavy oil, it is most general to apply an external force such as centrifugal force to the heavy oil for separating the floating particles and the heavy oil by the difference of densities. However, owing to the extreme fineness of the floating solid particles, it is impossible to sufficiently remove them from the heavy oil by simply subjecting the heavy oil to centrifugation. Accordingly, so far as the separation is carried out depending on the gravitational difference between the floating particles and the heavy oil, it becomes necessary to coagulate or agglomerate the minute particles into far larger particles.
A number of methods for removing the minute quinoline-insoluble component based on the principle have been hitherto proposed as follows:
(1) The method of thermal treatment of the heavy oil for bringing the size of the quinoline-insoluble solid particles larger and of removing the enlarged particles.
(2) The method of adding a heavy oil of petroleum origin to a heavy oil of coal origin, thereby adhering a high polymeric component (so-called gum-like component) to the quinoline-insoluble minute solid impurities to enlarge the size of the floating particles and if necessary, admixing an aromatic or aliphatic solvent therewith followed by stirring the resultant mixture while heating or by cooling the resultant mixture to separate and remove the thus formed insoluble precipitate (refer to Japanese Patent Applications Laying Open No. 55-104387 (1980) and No. 55-113606 (1980)).
(3) The method of admixing an organic solvent with the heavy oil, thereby agglomerating the minute insoluble precipitating material containing the quinoline-insoluble component into larger particles to separate and remove thereof (refer to Japanese Patent Applications Laying Open No. 55-136111 (1980), No. 56-49791 (1981) and No. 56-59611 (1981)).
However, the methods hitherto proposed are not the practically applicable and effective method because of the following reasons:
Namely, in the method (1), since the separated insoluble precipitate is extremely small in size, the speed with which the particles are separated is small, and the particles clog the mesh of filter-net on filtration resulting in the low efficiency of separation of insoluble precipitate. In addition, since it is necessary to carry out the separation or filtration at a high temperature in order to reduce the viscosity of the heavy oil to be treated, the method necessitates the high cost of installation and of operation which causes economic problem. In the method (2), since the formation of the insoluble precipitate takes a long time period at an ordinary temperature, the process necessitates a thermal treatment at a temperature as high as 200.degree. C., stirring for a time period as long as several hours, a large amount of an expensive solvent and an apparatus for recovering the solvent, and accordingly, the method (2) is lacking in industrial efficiency and economic efficiency. In the method (3), the necessary amount of the organic solvent is 10 to 100 times of the amount of the heavy oil to be treated resulting in a very high cost of treatment. In addition, as in the method (2), the method (3) necessitates the apparata for cooling and leaving the thermally treated heavy oil under agitation to stand and for recovering and recycling the expensive solvent.
Furthermore, in the conventional method, some components of the heavy oil are brought into polycondensation by the heating of the heavy oil to cause the alteration of physical properties of the heavy oil to a large extent, and since those excessively polycondensed are removed together with the minute solid impurities, eventually the yield of carbonization of the product is reduced. This is one of the demerits of the conventional method.
It has been found by the present inventors that the quinoline-insoluble minute solid impurities can be removed effectively by adding a surfactant to the heavy oil and gently stirring the mixture, thereby agglomerating the minute solid impurities into far larger particles while utilizing the agglomerating effect of the surfactant instead of agglomerating the minute solid impurities by the formation of gum-like component according to the conventional method, and applying a centrifugal force with in the ordinary range to the heavy oil containing the thus formed larger particles.
It has been also found by the present inventors that in the case where an organic solvent of a boiling point of lower than 150.degree. C. such as benzene is added to the heavy oil in advance of the addition of or together with the surfactant to the heavy oil, the viscosity of the thus obtained mixture is fairly reduced to facilitate the admixture of the surfactant and the gentle stirring of the thus obtained mixture thus facilitating the formation of the larger particles.
The thus added organic solvent is easily removed by simple distillation after removing the larger particles, and is used in circulation.
In the case where a commercialized surfactant is applied together with or without the organic solvent for the separation of the quinoline-insoluble minute solid impurities, it has been also found by the present inventors that it is necessary to fulfill the specified conditions of agitation or stirring when the surfactant is admixed with the heavy oil or the mixture of the heavy oil and the organic solvent in order to obtain the practically ideal separation of the minute solid impurities from the heavy oil.
The object of the present invention is to obtain a suitable raw material containing smaller amount of quinoline-insoluble solid impurities than in the concentrationally treated raw material for producing carbon material such as carbon fiber from heavy oils of petroleum origin or of coal origin .